1. Field of the Invention
The present invention relates to a voltage controlled oscillator used for generating a local oscillation signal of a radio communication device, and the like, and a PLL circuit and a radio communication apparatus using the same.
2. Description of the Background Art
A voltage controlled oscillator (VCO) is widespread as a device for generating a local oscillation signal of a radio communication device. FIG. 14 is a diagram illustrating an exemplary configuration of a conventional voltage controlled oscillator. The conventional voltage controlled oscillator includes: an inductor 604a and an inductor 604b; a variable capacitance element 605a and a variable capacitance element 605b; an oscillation transistor 603a and an oscillation transistor 603b; and a current source 601. In FIG. 14, a bias circuit and the like are not shown.
The inductor 604a and the inductor 604b, and the variable capacitance element 605a and the variable capacitance element 605b form a parallel resonant circuit. A capacitance value of the variable capacitance element 605a depends on a voltage between both terminals of the variable capacitance element 605a, and a capacitance value of the variable capacitance element 605b depends on a voltage between both terminals of the variable capacitance element 605b. Specifically, a capacitance value of each of the variable capacitance element 605a and the variable capacitance element 605b depends on a control voltage Vt applied to a frequency control terminal 602 from an external circuit. As a result, a resonant frequency of the parallel resonant circuit is variable. The conventional voltage controlled oscillator oscillates in the neighborhood of a resonant frequency of the parallel resonant circuit, and therefore it is possible, by adjusting control voltage Vt, to control the oscillation frequency of the voltage-controlled oscillator to be a desired frequency. The oscillation transistor 603a and the oscillation transistor 603b each generate a negative resistance so as to cancel a loss caused due to a parasitic resistance component of the parallel resonant circuit, thereby satisfying an oscillation condition.
A relationship between a control voltage for the voltage controlled oscillator and an oscillation frequency thereof substantially depends on characteristic of a variable capacitance element. Therefore, it is preferable to use a variable capacitance element which allows its capacitance to gradually change in a wide range of control voltage. That is, it is preferable that the oscillation frequency linearly changes in a wide range of control voltage Vt.
The reason is as follows. When the conventional voltage controlled oscillator is used so as to form a PLL (phase-locked loop) circuit, transient response characteristic and/or noise band characteristic of the PLL circuit depend on a frequency sensitivity (a rate at which the oscillation frequency changes against the control voltage Vt). Therefore, when a frequency sensitivity changes in accordance with a frequency (that is, when a frequency nonlinearly changes), the characteristic of the PLL circuit itself changes in accordance with the frequency. Further, when a frequency sensitivity against the control voltage Vt is high, the frequency changes due to even a substantially low noise applied to the frequency control terminal 602, whereby a problem arises that phase noise characteristic is deteriorated.
When the conventional voltage controlled oscillator described above is provided on a semiconductor substrate by using a particular process for forming the variable capacitance element 605a and the variable capacitance element 605b, cost is increased. Therefore, it is difficult to realize the use of the variable capacitance element improving linear characteristic. FIG. 15A shows a symbol representing an inversion-type variable capacitance element using a gate capacitance between a gate terminal and a terminal connected to a drain terminal and a source terminal, the inversion-type variable capacitance element being widespread for CMOS process. FIG. 15B shows a change in the gate capacitance obtained when a reference voltage Vref is applied to the gate terminal of a MOS transistor, and the control voltage Vt is applied to the drain terminal and the source terminal.
Thus, in the variable capacitance element using the gate capacitance of the MOS transistor which is typically used, a capacitance value rapidly changes in the neighborhood of a threshold voltage (voltage Vth in FIG. 15B), so that the oscillation frequency also rapidly changes in the neighborhood of the threshold value. As a result, a problem arises that transient response characteristic and/or noise band characteristic of the PLL circuit using the conventional voltage controlled oscillator substantially change depending on a frequency.
In order to solve the problems, the following circuit has been already suggested.
FIG. 16 is a diagram illustrating a voltage controlled oscillator using a technique for improving linear characteristic represented by a conventional variable capacitance element (see, for example, U.S. Pat. No. 6,995,626).
The conventional voltage controlled oscillator shown in FIG. 16 includes: inductors 604a and 604b; variable capacitance elements 605a, 605b, 606a, 606b, 607a, and 607b; DC cut capacitive elements 608a, 608b, 609a, 609b, 610a, and 610b each of which cut a direct current; high frequency interruption resistances 611a, 611b, 612a, 612b, 613a, and 613b; oscillation transistors 603a and 603b; and a current source 601. In FIG. 16, the same components as shown in FIG. 14 are denoted by the same corresponding reference numerals as used for FIG. 14, and no description is given for the same components.
The variable capacitance elements 605a and 605b, and the DC cut capacitive elements 608a and 608b form a variable capacitance circuit A. The variable capacitance elements 606a and 606b, and the DC cut capacitive elements 609a and 609b form a variable capacitance circuit B. The variable capacitance elements 607a and 607b, and the DC cut capacitive elements 610a and 610b form a variable capacitance circuit C. The variable capacitance elements 605a, 605b, 606a, 606b, 607a, and 607b are each an inversion-type MOS transistor using a capacitance between a gate terminal and a terminal connected to a drain terminal and a source terminal, the inversion-type MOS transistor being used for CMOS process. Capacitance values of the variable capacitance elements of the variable capacitance circuits A, B, and C change in accordance with a control voltage Vt applied to a frequency control terminal 602 and reference voltages Vref1, Vref2, and Vref3, respectively. Here, the reference voltage Vref1 is inputted to a connection point between the variable capacitance element 605a and the DC cut capacitive element 608a, and a connection point between the variable capacitance element 605b and the DC cut capacitive element 608b. The reference voltage Vref2 is inputted to a connection point between the variable capacitance element 606a and the DC cut capacitive element 609a, and a connection point between the variable capacitance element 606b and the DC cut capacitive element 609b. The reference voltage Vref3 is inputted to a connection point between the variable capacitance element 607a and the DC cut capacitive element 610a, and a connection point between the variable capacitance element 607b and the DC cut capacitive element 610b. As a result, a resonant frequency of a parallel resonant circuit is variable.
When the reference voltages Vref1, Vref2, and Vref3 are different at intervals of voltage Vd, characteristics of the variable capacitance circuits A, B, and C change against the control voltage Vt, as shown in FIG. 17A, at intervals of Vd (160 mV in FIG. 17A). The capacitance of the parallel resonant circuit is a sum (that is, a total capacitance) of the capacitances of the three variable capacitance circuits A, B, and C, and the total capacitance represents the characteristic indicated by the dashed-dotted line in FIG. 17B. That is, the capacitance is allowed to gradually change against the control voltage Vt.
The oscillation frequency fo of the voltage controlled oscillator is obtained in accordance with equation [1] as follows when an inductance of an inductor of the parallel resonant circuit is represented as L, the capacitance value of the variable capacitance circuits A, B, and C is represented as Cv, and a capacitance value of a parasitic capacitance generated in a negative resistance circuit or the like is represented as C.fo=1/(2π×√{square root over (L×(Cv+C)))}  [1]
When equation [1] is solved for obtaining the capacitance value Cv of the variable capacitance circuits A, B, and C, equation [2] is obtained as follows.Cv=C−1/(4π2Lfo2)  [2]
The inductance L and the capacitance value C of the parasitic capacitance each represents a constant value. Therefore, it is necessary to change the capacitance value Cv in proportion to 1/(fo2), instead of linearly changing the capacitance value Cv of the variable capacitance circuits A, B, and C, so as to linearly change the oscillation frequency fo against the control voltage Vt.
In the conventional improvement method described above, however, it is possible to improve the linear characteristic represented by the capacitance changing in a wide range of control voltage by gradually changing the capacitance against the control voltage Vt, whereas the improvement of the linear characteristic represented by the oscillation frequency of the voltage controlled oscillator changing in a wide range of control voltage is limited.
Further, when the number of variable capacitance circuits to be provided in parallel with each other is increased, the linear characteristic represented by the capacitance of the variable capacitance circuit can be improved. However, an area of a semiconductor chip needs to be increased and/or a layout is complicated, and therefore it is necessary to limit the number of the variable capacitance circuits connected in parallel with each other, whereby it is difficult to improve the linear characteristic represented by the frequency changing in a wide range of control voltage.